Fuel cell power plants are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus. In such power plants, one or typically a plurality, of planar fuel cells are arranged in a fuel cell stack. Each cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. The reducing fluid and the oxidant are typically delivered to and removed from the cell stack via respective manifolds. In a cell using a proton exchange membrane (PEM) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes, depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is a PEM electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials, is fixed and cannot be leached from the cell, and has a relatively stable capacity for water retention.
In operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode including water resulting from proton drag through the PEM electrolyte and rates at which water is removed from the cathode and at which water is supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into an external circuit varies and as an operating environment of the cell varies. For PEM cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out, thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded, thereby effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode, the PEM may dry out, limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance.
As fuel cells have been integrated into power plants for powering vehicles, maintaining a water balance within the power plant has become a greater challenge because of a variety of factors. To minimize weight and space requirements on a vehicle, the plant must be self-sufficient in water to be viable. That means that enough water must be retained within the plant to offset water losses from gaseous streams of reactant fluids passing through the plant. For example, any water exiting the plant through a cathode exhaust stream of gaseous oxidant or through an anode exhaust stream of gaseous reducing fluid must be balanced by water produced electrochemically at the cathode and retained within the plant. Otherwise, one incurs the cost and size/weight penalty of various water recovery components if it is necessary to make water.
An additional requirement for maintaining water self-sufficiency in fuel cell power plants is associated with components necessary to convert hydrocarbon fuel to a stream of hydrogen-rich reducing fluid for the anode. Those components may include a reformer that relies on steam to convert the hydrocarbon to the desired reducing fluid. The fuel processing components or system water and energy requirements are part of an overall water balance and energy requirement of the fuel cell power plant. Water made into steam in a boiler must be replaced by water recovered from the plant such as by condensing heat exchangers in the cathode exhaust stream and associated piping.
A common approach to enhancing water recovery and retention is use of condensing heat exchangers in exhaust streams of the power plant wherein the exhaust streams are cooled to a temperature at or below their dew points to precipitate liquid water, which is then returned to the power plant. Typically, such a condensing heat exchanger is used to cool a cathode exhaust stream, which upon leaving a cathode chamber includes evaporated product water. The condensing heat exchanger passes the cathode exhaust stream in heat exchange relationship with a stream of cooling ambient air, and then directs condensed water indirectly through piping back to water storage.
While condensing heat exchangers have enhanced the water recovery and energy efficiency of fuel cell power plants, the heat exchangers encounter decreasing water recovery efficiency as ambient temperature increases. Where the power plant is to power a vehicle, such as an automobile, the plant will be exposed to an extremely wide range of ambient temperatures. For example, where an ambient air coolant stream passes through a heat exchanger, performance of the exchanger will vary as a direct function (inversely) of the temperature of the ambient air because decreasing amounts of liquid precipitate out of power plant exhaust streams as the ambient air temperature increases.
An additional requirement of using such condensing heat exchangers in fuel cell power plants powering vehicles is related to operation of the vehicles in temperatures below the freezing temperature of water. Because water from such exchangers is often re-introduced into the PEM fuel cells of the plant, the water may not be mixed with conventional antifreeze to lower its freezing temperature because such antifreeze would be absorbed by the catalysts in the cells and thereby decrease cell efficiency.
To overcome some of the aforementioned limitations which exist if the power plant were to use a condensing heat exchanger, a recent development instead uses an energy recovery device (ERD) that employs a fine pore enthalpy exchange barrier for effective transfer of energy and/or water from one gas stream to another over a broader range of ambient air temperatures and conditions. An example of such an ERD having a fine pore enthalpy exchange barrier between the inlet oxidant gas flow path and the fuel cell exhaust gas flow path is shown and described in U.S. Pat. No. 6,274,259 that issued on Aug. 14, 2001 to Grasso, et al, and is assigned to the assignee of the present invention, and which patent is hereby incorporated herein by reference. The fine pore enthalpy exchange barrier may be viewed as comprising one or more plates that include a support matrix of porous material such as fiber and/or particulate material, with porous graphite layers perhaps being preferred. The support matrix defines pores, which pores are then filled with a liquid transfer medium such as an aqueous solution or the like, to create a gas barrier. The pores of the support matrix have a size range of about 0.1–100 microns and the matrix is hydrophilic so as to be wetted by the liquid transfer medium and result in a bubble pressure typically greater than 0.2 p.s.i. An inlet surface of the fine pore enthalpy exchange barrier is positioned in contact with a process oxidant inlet stream entering a fuel cell power plant, and an opposed exhaust surface of the barrier is positioned in contact with an exhaust stream exiting the plant so water and heat exchange from the exhaust stream directly into the process oxidant inlet stream. The liquid transfer medium, which may be a low volatility, aqueous solution having a low freezing temperature, may simply be trapped in the fine pore enthalpy exchange barrier in an adequate amount or, if necessary, may be re-supplied from a supply reservoir. This assures a gas barrier between the two gas flow streams and allows some humidification of the process oxidant inlet stream as its ambient temperature rises.
While affording significant benefit over the condensing heat exchangers of the prior art, the ERD described in the aforementioned U. S. Pat. No. 6,274,259 may be too responsive to changes in the ambient humidity and/or temperature levels of the process oxidant inlet stream. Very hot, dry, i.e., “arid” inlet air may cause the evaporation of humidifying water to occur at a higher rate than can provided by the condensing exhaust gas stream; possibly exceeding the total level of fuel cell-generated recoverable exhaust water. This situation is unacceptable because it can result in dry-out of the ERD plates, particularly along the inlet air leading edge of the plates.
Accordingly, what is needed is an improved arrangement for control of humidification in, and of, an energy recovery device in a fuel cell power plant, particularly with respect to the process oxidant inlet stream flowing therethrough.
Further desirable is an arrangement for controlling performance of an energy recovery device in a fuel cell power plant to regulate water balance in the power plant.
Still further desirable is the ability to rapidly and simply control humidity and/or temperature conditions associated with a fine pore enthalpy exchange barrier in an energy recovery device of a fuel cell power plant.